[web] Introduce Members and Sidebars thread menu actions

Summary:
Added Members and Sidebars menu items, that are displayed conditionally.
The onClick functionality will be added in one of next diffs.

Test Plan: Checked if Sidebar item appears correclty after adding sidebars to thread, and if Memberes is not displayed in Personal threads.

Reviewers: palys-swm, benschac, atul, ashoat

Reviewed By: palys-swm, ashoat

Subscribers: ashoat, Adrian, atul, karol-bisztyga

Differential Revision: https://phabricator.ashoat.com/D3190